Final Report

Background
The importance of coastal groundwater discharge in delivering dissolved
nutrients, such as nitrate and phosphate, to coastal waters has
often been overlooked, primarily because it is difficult to estimate
(Johannes, 1980; Nixon et al., 1986; Simmons, 1992). The problem
lies in the fact that the flow of groundwater through coastal marine
sediments, called submarine groundwater discharge (SGWD), is difficult
to quantify using traditional methods such as seepage meters since
the discharge is often patchy and may vary with time. Unlike rivers,
submarine fluid discharge bypasses the estuary filter, which is
an important mechanism for contaminant removal in many coastal settings
(Moore and Shaw, 1998). Even if SGWD rates are modest, dissolved
nutrient concentrations in groundwater may be sufficiently high
to have a significant impact on the nutrient budgets for receiving
waters. Recently, radium has been shown to be a useful chemical
indicator of SGWD and, having four isotopes with half-lives ranging
from four days to 1600 years, can be used to estimate rates of SGWD
on a wide range of time-scales (Moore, 1996; Rama and Moore, 1996).

A key biogeochemical problem associated with coastal groundwater
flow on Cape Cod is the introduction of "new" nitrogen entrained
by groundwater plumes as they pass through septic tank fields located
along the coastline (Valiela et al., 1992; Weiskel and Howes, 1991).
As a result, nitrate concentrations may be several orders of magnitude
greater than the receiving waters (Valiela et al., 1990, 1992; Andrews
et al., 1999). Here, we present a study of SGWD in Waquoit Bay,
MA utilizing radium isotopes as tracers of SGWD-derived dissolved
inorganic nitrogen (DIN) flux to the estuary. Lastly, we compare
these results with productivity estimates from the literature in
an attempt to determine if the estuary is a net source of nutrients
to coastal waters.

Study Area
Waquoit Bay is an enclosed estuary located on the south shoreline
of Cape Cod, MA. Its watershed comprises nearly 65 km2
extending roughly 10 km north from the head of the bay. The bay
on average is relatively shallow with a mean depth of 1 m; major
freshwater sources include the Quashnet River (to the east) and
the Childs River (to the west). In terms of the total freshwater
budget, these rivers are a minor component compared with direct
groundwater discharge (Cambareri and Eichner, 1998). Surrounded
by a population of over 8,000 year-round residents (not including
summer residents), groundwater-derived nutrients from private septic
systems have led to an increasing number of eutrophication events
in this watershed (Valiela et al., 1992)

Eutrophication in Waquoit Bay has been directly linked to sewage-derived
nitrogen inputs via direct groundwater discharge to the estuary
(Valiela et al., 1990; McClelland et al., 1997). The coarse, unconsolidated
sands that characterize this watershed contribute significantly
to the rapid transport of nutrients to coastal waters. The most
notable ecological impact has been a decline in shellfish population
and sea grass coverage. The former is likely due to seasonal the
seasonal decline in dissolved oxygen. The latter is caused by secondary
effects, namely an increase in epiphytes which intercept light from
their growth substrate, eel grass blades (Valiela et al., 1992)

Summary of Results
Due to rapid increases in population, anthropogenic sources of nitrogen
have adversely impacted the water quality of coastal ponds on Cape
Cod. A major source of "new" nitrogen to these estuaries is groundwater,
which intercepts septic tank fields in its flow path to the coastline.
Many attempts have been made to quantify this process; however,
groundwater discharge is often patchy in nature and therefore difficult
to study using traditional techniques such as seepage meters. In
Waquoit Bay, MA, we tested an approach based on radium, which is
naturally enriched in aquifer fluids and has four isotopes with
half-lives ranging from 4 days to 1600 years. Groundwater entering
the bay was low in salinity and contained several orders of magnitude
greater radium and dissolved inorganic nitrogen (DIN) relative to
ambient bay water. Using a mass-balance approach for radium, we
calculated a submarine groundwater flux of ~28,000 m3 d-1, which
compared well with aquifer recharge rates calculated from rainfall.
From the DIN content of the groundwater, we estimated that ~1500
mol N d-1 was directly input to the estuary. However, this nitrogen
flux was small in comparison to DIN fluxes from the heavily populated
subestuaries. Furthermore, our results suggest that groundwater
flux of DIN was assimilated by plant biomass during the summer but
may be exported from the embayment to coastal waters during the
winter months.

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